This invention relates generally to offshore drilling and processing facilities, and more particularly to apparatus and methods for making firm and level foundations for said facilities, and for providing support for said facilities above the sea floor to increase their operational water depth capabilities.
In the offshore drilling and/or processing of hydrocarbons or other substances, it is often desired or required that drilling and/or processing equipment be supported on a firm and level subsea foundation. Often, too, such a firm and level subsea foundation must be raised a distance above the level of the natural sea floor in order to allow such equipment to operate in deeper water. Sometimes, the natural seafloor can be leveled, if need be, perparatory to setting a subsea platform foundation upon it as shown, for example, in U.S. Pat. No. 2,940,266, issued June 14, 1960 to Smith. However, such leveling of the sea floor is often difficult or dangerous and is generally time-consuming and expensive, especially for platforms that are moved frequently. Other problems in obtaining a level subsea foundation may arise due to adverse subsea soil conditions at or around the offshore facility location. For example, the sea floor may be rocky or uneven, causing a subsea platform or support resting upon it to tilt or be unstable, or it may be composed of a soft mud that is unable to support the weight of the offshore facility. In addition, the offshore location may be in a mud slide area that will require some method of soil stabilization.
Various approaches have been taken in the past toward providing a firm, stabilized, level or raised foundation for offshore drilling and/or production facilities. One approach is to divide the offshore apparatus into a plurality of sections which are pivotally linked together, the bottom of each section being free to assume inclinations conforming to different areas of the sea floor. See, e.g., U.S. Pat. No. Re. 24,346, issued Aug. 20, 1957, to Dawson. Where the sea floor is soft, caissons may be driven into the mud to provide support and stability. See, e.g., U.S. Pat. No. 4,045,968, issued Sept. 6, 1977, to Gerwick, Jr. However, both types off the above devices are complex and expensive to manufacture, install and maintain.
In some cases, oyster shell or gravel is dropped on the sea floor forming a mound on which to set a drilling and/or production platform. The results are usually poor due to the oyster shell or gravel being washed away with the current, or it may flatten out or collapse into itself. Jack-up devices are also used to jack up an underwater platform, but they are expensive and it is often difficult or impossible to get any bottom pressure on the spuds in certain areas.
There is also an item known as a prefabricated floatable flexible barrier. This barrier is towed out to the drilling site, towed around the wellhead or platform, sunk, secured into place with pilings, filled with sand or other material, and then the drill barge is brought into the barrier and placed on the sand. After the drill barge is brought into place, its doors are closed. See, e.g. U.S. Pat. No. 2,939,290, issued June 7, 1960 to Crake. One of the problems with this system is that it is difficult to tow since it is not in a rigid form. It will move with the wind, waves or current and may not be controllable under tow. Furthermore, when it arrives at the wellhead and is sunk, the current may move it. Also, after it has been sunk, it will be difficult to maneuver around the wellhead because it will either be stuck in the mud or it may drift in the current. Moreover, if the bottom has a slope to it, the flexible barrier will not have any support to hold the additional sand required on the low side. The barrier must be secured with pilings to work at all. Piling is an expensive operation offshore. Pilings are also expensive to remove. Another problem with this barrier is that it cannot transport its load of sand to another location and if the sand is left in an area, it could block a channel or disrupt the environment in some other way. Furthermore, the many hinges and flexible hoses cause problems with breakdowns which in turn can shut down the operation or cause the form to sink while under tow.
Artificial islands have also been used in the past. These islands are designed to be towed out to a location, sunk, filled with sand or other material and then the drilling and production equipment is placed on them. See, e.g., U.S. Pat. No. 3,740,956, issued June 26, 1973 to Guy et al. This type of device is not designed to be sunk completely under water and then put into operation and later recovered because it does not have a sealed top on it that will allow it to be filled with air when it is completely submerged. It is open at the top and it is often open at the bottom. Accordingly, it cannot act as an enclosed tank to hold air. Thus, it would not be possible to recover such a device when it is completely submerged. A structure that is the size of these artificial islands is also too large to be lifted out of the water by a derrick barge.